Raptor observations of delayed explosive activity in the high-redshift gamma-ray burst GRB 060206

The Rapid Telescopes for Optical Response (RAPTOR) system at Los Alamos National Laboratory observed GRB 060206 starting 48.1 minutes after γ-ray emission triggered the Burst Alert Telescope on board the Swift satellite. The afterglow light curve measured by RAPTOR shows a spectacular rebrightening by ~1 mag about 1 hr after the trigger and peaks at R ~ 16.4 mag. Shortly after the onset of the explosive rebrightening, the optical transient doubled its flux on a timescale of about 4 minutes. The total A-band fluence received from GRB 060206 during this episode is 2.3 × 10^-9 ergs cm^-2. In the rest frame of the burst (z = 4.045), this yields an isotropic equivalent energy release of E_iso ~ 0.7 × 10⁵⁰ ergs in just a narrow UV band, λ ≃ 130 ±22 nm. We discuss the implications of RAPTOR observations for untriggered searches for fast optical transients and studies of GRB environments at high redshift.